The field of art to which this invention pertains is water-soluble polymers.
Polygalactomannans, particularly the naturally occurring guar gum and locust bean gum, have been known for some time and have found many industrial uses. To improve the performance of these gums and to widen their fields of use, polygalactomannans have been modified by reaction with various reagents to form derivatives of the gums. Examples of such derivatives are hydroxyalkyl ethers of polygalactomannans, aminoalkyl ethers of polygalactomannans, mixed derivatives which are carboxyalkyl-hydroxyalkyl ethers and carboxyalkyl-aminoalkyl ethers of polygalactomannans.
Hydroxyalkyl ethers of polygalactomannans are described in U.S. Pat. No. 3,326,890. Aminoalkyl ethers of the gums are disclosed in U.S. Pat. No. 3,498,912. Carboxyalkyl-hydroxyalkyl mixed derivatives are described in U.S. Pat. No. 3,723,409. Mixed derivatives containing both anionic groups, e.g., carboxyalkyl, and cationic groups, e.g., aminoalkyl, are disclosed in U.S. Pat. No. 3,467,647.
An important use for polygalactomannans and their derivatives is as thickeners for industrial applications. However, after the purpose for using the thickened aqueous fluid has been served, it is often desirable to lower the viscosity of the fluid. This lowering of viscosity is accomplished by using a "breaker" which acts on the water-soluble polymer to reduce its molecular weight and to thin the fluid in which it is dissolved.
The breaking of aqueous gels or thickened aqueous fluids has been accomplished by various techniques. One such method is to use the degradative action of acids, enzymes, or mild oxidizing agents as disclosed in U.S. Pat. No. 4,169,791. In U.S. Pat. No. 4,464,268, sodium hypochlorite is described as being a drastic treating agent for degrading water-soluble polymers. The use of ammonium and alkali metal persulfates in combination with tertiary amines is disclosed in U.S. Pat. No. 4,250,044 as being a suitable beaker system for high viscosity fluids. Lithium hypochlorite is described as a breaker material in Oil and Gas Journal, Dec. 12, 1983, pp. 96-101. The thickening of aqueous fluids and various techniques for reducing the viscosity of the fluids is described in such patents as U.S. Nos. 3,002,960; 3,779,914; 4,144,179; and 4,552,668. As described in U.S. Pat. No. 4,654,043, starch sizing agents are degraded by treatment with hypochlorites and amino compounds.
In commonly assigned U.S. Pat. No. 4,647,385, the viscosity of thickened aqueous solutions is reduced by the addition of alkali metal and alkaline earth metal salts of hypochlorous acid plus tertiary amines. The combination of the salt and the tertiary amine is much more efficient in speed of reduction in viscosity and in the completeness of the degradation of the polymer than the use of the metal salt alone.
Thickened aqueous fluids are useful in oil recovery processes, particularly in the practice of fracturing subterranean formations. In the fracturing operations, a thickened fluid having granular insoluble particulate material suspended therein, is injected down a well penetrating the formation under hydraulic pressure. The pressurized fluid causes the formation to crack or fracture. The suspended insoluble granular particles are forced into the fractures and prop open the fractures when the fracturing pressures are released. Before the well is put back into operation, the thickened fluid must be removed from the well. Such removal is facilitated if the viscosity can be quickly reduced.
Industry is constantly searching for improved and efficient thickeners for aqueous fluids and for more efficient breaker systems.